A switching power supply, of the kind mentioned above is disclosed in West German Patent Publication DE-OS 36 37 560. In this prior switching power supply, the battery branch circuit is switched on in the event of a mains a.c. voltage failure so that the functions of a load can be sustained over a certain period after the mains failure. A special monitoring circuit is provided therefor which supplies control pulses either to the first or to the second switching transistor dependent on the existence of mains voltage. When the control pulses are transferred to the second transistor, the current pulses generated with it in the battery branch circuit are directly transferred to an output filter or to the load.
This known principle of an emergency power supply in switching power supplies requires an enormous expenditure to provide the necessary control and energy transfer. When several output voltages are generated, apart from the monitoring circuit for the switching of the control pulses from a first to a second control line, each single output voltage requires a special battery branch circuit to sustain it during mains failure.